During the fabrication of interconnect structures many types of components are attached to circuit cards. The wiring layers on the card as well as the components generate resistance heat. Furthermore, active components generate heat during switching. The heat raises the temperature of the components above the ambient temperatures of the air surrounding the systems. Most electronic components have some maximum temperature above which they will quickly fail, but also, the life span of active components is generally related to the temperature raised to some power (typically 2). Thus it is advantageous to keep electronic components as cool as practical.
For most applications, most of the heat generated in the components is conducted by a thermal path through the component terminals into a circuitized card substrate (most commonly fiberglass filled epoxy) and through the wiring layers of the card. The card dissipates the heat by convection to the surrounding air, and by radiation to the enclosure containing the board, and through card connectors and supports to other boards or the enclosure. Also, typically, some of the generated heat is removed through the back side of the component by convection to the surrounding atmosphere and to a lesser extent, radiation to any cooler surfaces surrounding the card.
Recently, component densities on circuit boards have increased to the extent that a large portion of the space on the card is heat generating components, also line widths on circuit boards are narrowing so that more heat is generated in the signal lines of the circuitized cards. Thus, the thermal path through the leads is less useful in lowering component temperatures and the thermal path through the back sides of components is becoming more important.
In very high performance applications (e.g. large mainframes) a helium or water cooling system including a cold plate, is provided and the cold plate is thermally connected to the components by thermal grease. Such thermal greases are typically thick organic liquids (such as silicone oils) filled with particles of a highly thermoconductive material such as alumina, beryllium, copper, silver, or graphite. In main frame applications flip chips have been attached on the backs of ceramic PGAs (CPGAs) using controlled collapse chip connections C4s. The chips are directly thermoconnected to a cold plate of a liquid cooling system by a thermal grease.
In more typical high performance applications a heat sink (e.g. aluminum Al or copper Cu) is mechanically clipped or screw attached against a component to increase the dissipation of heat by convection directly into the surrounding air. Thermal grease is usually provided to fill voids between the heat sink and the component since even a very thin air gap becomes a significant resistance layer in the thermal path.
To increase thermal performance, individual power transistors have been supplied potted in T0-T3 cans by silicone or epoxy potting compound. Again such adhesives have been filled with thermoconductive materials to increase thermal conduction and reduce the temperature of the semiconductor devices. Such discrete power transistors have also been supplied in a small thin package with a hole for screwed connection to circuit boards with thermal grease provided between the transistor and board. Also, simple heat sinks of bent plate, have been provided to increase thermal performance.
Integrated circuit components are characterized by large numbers of input and output I/O connection terminals. Organic materials have been used extensively in production of such integrated electronic components. Most common components are manufactured by producing a lead frame on a polyimide tape, using an adhesive (e.g. epoxy or silicone adhesives) to connect the back side of a wire bond chip onto the lead frame tape and wire bonding to connect the front of the die to the leads of the frame. The adhesives manufactured for connecting chips to polyimide tape are usually filled with thermoconductive particles to increase heat conduction from the chip and thus minimize chip temperature.
Plastic components are usually manufactured by transfer molding to encapsulate the chip, bond wires, and part of the leads with epoxy to form a plastic substrate. Ceramic components are usually manufactured by providing a cavity under a ceramic substrate into which the chip, bond wires are placed and then the polyimide tape is adhesively bonded to the ceramic substrate with epoxy or silicone adhesives and the chip and cavity is filled with thermoconductive epoxy or silicone adhesive. A ceramic bottom substrate may be bonded over the bottom of the cavity to protect the bottom of the component.
Heat sinks have usually been connected to integrated packages by screwing or clipping. Non-silicone greases are commonly used between the heat sink and the modules to increase thermal performance. The heat sinks are usually connected to integrated components after circuit board assembly. Recently in order to reduce costs, manufacturers have started adhesively bonding heat sinks directly to organic or ceramic surfaces of electronic components after circuit board assembly using epoxy based materials. Since the coefficient of thermal expansion CTE of epoxy is around 50-80 ppm/.degree. C. and the CTE of Al is 23.4 ppm/.degree. C. and copper is 17 ppm/.degree. C. and the coatings used for heat sinks (bare copper, nickel coated copper, bare Al, anodized Al, chromium conversion on Al) are smooth and do not adhere well to epoxy, it is difficult to maintain bonding during thermal cycling. Typically in order to increase thermal conduction and to also reduce delamination problems, the epoxy used for heat sink attach is filled with thermoconductive metal or ceramic particles that modify the CTE of the adhesive to a level (e.g. 30-40 ppm/.degree. C.) which is midway between that of the module material (epoxy) and that of the base metal of the heat sink.
In high temperature applications silicone adhesives have been suggested to connect anodized Al heat sinks to ceramic surfaces of modules because silicone-based adhesives are very heat resistant. Silicone-based adhesives have Tg's below 25.degree. C. and Young's Module below 2,000 psi, and are thus, very compliant. However silicone has a very high coefficient of thermal expansion of around 200 ppm, and it is difficult to provide reliable bonds between silicone and common heat sink surfaces. Mechanically, the bond strengths between silicone adhesive and these metal surfaces are about one third to one half the strength of epoxy. Silicone is a weak material with a tensile strength of only about 500 psi verses about 2,000 psi for epoxy. Also, constituents of silicone adhesives have a tendency to migrate out contaminating surfaces with a micro thin coating which prevents subsequent attachment of other organic materials to the circuit boards such as photoresists, solder resists, and encapsulants. Usually heat sinks are attached after other process for constructing circuit boards is complete thus minimizing contamination problems during construction. Contamination is especially a problem for reworked components because heating the silicone during rework greatly increases the contamination and prevents any subsequent encapsulation or other non-silicone processes on the circuit board that may be required for rework.
Those skilled in the art are directed to the following citations for additional information. U.S. Pat. No. 4,000,509 to Jarvela describes using thermal grease to thermally connect a flip chip to a heat sink. U.S. Pat. No. 5,271,150 to Inasaka, describes manufacture of ceramic substrates. U.S. Pat. No. 4,604,644 to Beckham; U.S. Pat. Nos. 4,999,699 and 5,089,440 to Christie; and U.S. Pat. No. 4,701,482 to Itoh describe using epoxy to encapsulate C4 joints. U.S. Pat. No. 5,159,535 to Desai describes attaching the back of flip chips to a substrate. European Patent Application 93104433.3 to Bennett discloses a semiconductor die attached to a lead frame. U.S. Pat. No. 4,914,551 to Anschel discloses attaching a semiconductor chip to a heat spreader of silicone carbide (SiC), aluminum nitride (AlN), or Cu-Invar-Cu using epoxy. Removal of Heat From Direct Chip Attach Circuitry by G. Schrottke and D. J. Willson in IBM Technical Disclosure Bulletin Volume 32 Number 4A September, 1989, discloses using adhesive to bond silicone chips to a Cu-Invar-Cu heat spreader. U.S. Pat. No. 5,168,430 to Nitsch in FIG. 2 shows a hybrid circuit structure 3 cemented to a heat spreader 4. Technical Data Sheet for Ablebond P1-8971 by Ablestik of September 1993 recommends attaching a large die to a silver coated copper lead frame using flexible epoxy. Product Data Sheet for Prima-Bond EG 7655 by A.I. Technology, Inc. recommends bonding Alumina to Aluminum and Silicone to Copper. New Product Description for X3-6325 by Dow Corning 1994 suggests a silicone adhesive "used for bonding, sealing and adhering substrates lids and heat sinks." Information About High Technology Silicone Materials by Dow Corning 1988 suggest using SYLGARD.RTM. Q3-6605 silicone adhesive for attaching "hybrid integrated circuit substrates, components and devices to heat sinks." Semiconductor Packaging Materials Selector Guide by General Electric suggests using RTV6424 a silicone adhesive, "for die-attach applications" MS-523 Silicone Technical Bulletin by Thermoset Plastics, Inc. suggests using the silicone adhesive "to bond heat sinks to plastic, ceramic, and metalized packages." TC3280G Silicone Rubber Adhesive by General Electric 1994 describes the properties of the subject silicone adhesive. Exceptional Performance from the Development, Qualification and Implementations of a Silicone Adhesive for Bonding Heat sinks to Semiconductor Packages by Bosworth, Hsu, and Polcari in Proceedings of the 44th Electronic Components and Technology Conference, 1994 describes using silicone adhesive to connect heat sinks to ceramic packages. U.S. Pat. No. 5,180,625 to Wang, suggests connecting an aluminum plate to a ceramic circuit board using flexible thermoconductive epoxy. One-Part Thermal-Cure Silicone Adhesive by Vanwert and Wilson in Proceedings of the Technical Program NEPCON West 96 in Feb. 27-29 1996 at Anageim Calif., suggests using the silicone adhesive "for bare chip packaging." Thermoconductive Adhesives for High Thermally Stressed Assembly by Wilson, Norris, Scott and Costello suggests using I-4173 silicone adhesives for "bonding substrates, components and devices to heat sinks." U.S. Pat. No. 5,210,941 to Turek suggests attaching an aluminum heat sink to an organic circuit board using silicone adhesive. Japanese published application 64-148901 to Kanwa suggests a pagoda type heat sink. U.S. Pat. No. 4,092,697 to Spaight; and U.S. Pat. No. 4,742,024 to Sugimoto; U.S. Pat. No. 5,097,318 to Tanaka; U.S. Pat. No. 5,367,196 to Mahulikar; U.S. Pat. No. 5,168,430 to Nitsch; suggests thermally connecting a chip to a heat sink. U.S. Pat. No. 5,159,535 to Desai; U.S. Pat. No. 5,278,724 to Angulas; U.S. Pat. No. 5,147,084 to Behum suggest various flip chip carrying modules. Optimization of Thermal Adhesive Bond Strength for Extended use of TBGA at Elevated Temperatures by Bernier and Memis at NEPCON West Anaheim Calif., March 1995.
All the above citations are hereby incorporated in whole by reference to provide an enabling disclosure and to support the claims to which the applicant is entitled by law.